The invention relates to a method and apparatus for controlling moving material and the like, and, more particularly to improved method and apparatus for achieving a desired operating path of a web edge during movement of the web in a desired path of travel.
It is a common practice in manufacturing operations involving indefinite length of materials such as fabrics to transport the fabric in a desired path of travel for processing and/or collection. Typically, in textile manufacturing operations, textile fabrics in indefinite length form are longitudinally moved in a path of travel for treatment, inspection, and/or collection in roll form. In such operations, it is generally desirable that the fabric be secured close to its edges for effective processing and minimization of waste.
In the general operation of tenter frames, for example, fabric enters the machine at one end where it is engaged along its edges or selvages, by a series of upwardly extending pins which penetrate the fabric material. The pins are, in turn, secured to and supported by two endless tenter chains. The fabric is thus held along its opposite edges by the upwardly extending pins for the full period of travel through the tenter frame where it may be subjected to various treatments, for example, washing, drying, and dying.
Typically, tenter frames transversely maintain the fabric as its moves longitudinally through its path of travel. The selvages are typically reinforced to prevent the fabric edges from tearing where they are secured by the pins.
It is also desirable to secure the fabric near the selvages to reduce waste, since the selvage areas must often be trimmed from the finished fabric. One difficulty with consistently pinning the fabric at the selvages is that the fabric may transversely shift as it enters the machine. Additionally, a fabric edge itself may be uneven. If the tenter frame fails to pin an area of the fabric, a gap is produced, particularly when the fabric is stretched downstream. These gaps must be cut out from the finished material, causing waste, inefficient product handling and possibly machine down time.
Thus, while on one hand it is desirable to pin the fabric as close to the longitudinal edge as possible to prevent trimming waste, it is on the other hand desirable to ensure pinning of the fabric deep enough inward from the longitudinal edge to avoid defects inherent from mispinning.
Accordingly, the two tenter frame endless chains are typically at least partially supported on corresponding frame members that are transversely adjustable with respect to the path of travel of the moving fabric. These frame members may be adjusted according to the variation of the fabric edge, attempting to secure the fabric near its edges while avoiding mispins.
To control the transverse adjustment of a frame member, it is known to monitor the operating path of a corresponding edge, for example its transverse position, upstream from the point at which it is secured by the frame member and to adjust the frame member to compensate for variations of the operating edge path from a desired edge path. In a typical arrangement, a detecting device is mounted on the frame member and employs photosensor devices to detect the presence or absence of the fabric edge.
When the fabric edge deviates from the desired edge path defined, for example, by the placement and configuration of the photosensor devices, the control device initiates the adjustment of the frame member in an appropriate direction to accommodate the edge deviation and to ensure that the fabric is pinned at a generally consistent distance from the longitudinal edge. Thus, the operating edge path achieves the desired edge path.
One difficulty arising from the use of such systems results from color variation of fabrics often run on tenter frames. Color effects may be such that a control system might react too slowly, or not at all, when fabrics of certain colors are run on the tenter frame. Thus, for example, a tenter frame may tend to pin a lighter color fabric nearer its edge than a darker color.
One method of alleviating such problems associated with fabric color is to adjust the intensity of light emitted by the photosensor light source according to fabric color. Such a practice may be inefficient if such adjustments are frequently required. Additionally, light intensity increases may cause control system malfunctions due to reflections from background objects.
Another difficulty encountered with such control systems is that dust and fabric particles frequently collect on photosensor surfaces. This may inhibit photosensor operation and decrease the control system's effectiveness.
Furthermore, typical fabric edges are relatively uneven and may have strings and other abnormalities. Typical tenter frame control devices attempt to adjust the frame members according to the position of the edge. The edge variations may cause unnecessary and overly frequent frame member adjustment.
Furthermore, control systems generally adjust the speed at which corrections are made to frame member position according to the distance the edge deviates from a desired position. Such systems typically vary the correction speed proportionally to the position of the edge within the detection area of one or more photosensors. For example, frame member correction speed may increase as the fabric edge moves across the detection area. Greater speed variation may be achieved by adding photosensors on either side of the fabric edge desired position.
Such control systems may not react fast enough in response to gross fabric edge deviations. If they are configured to increase speed quickly as the fabric edge moves away from the desired position, the frame member may also move quickly as the edge returns, potentially causing the frame member to overshoot the desired position.
Another difficulty encountered in such arrangements arises from the conflict between the need to pin as close to the longitudinal edge as possible and the general tendency of textile machine operators to overcompensate against mispinning by pinning the fabric farther inward, or deeper, from the longitudinal edge. Operators may adjust the desired edge path, by adjusting the detecting device, to cause the pins on the frame member to pin the fabric at a consistently deep position with respect to the longitudinal edge. That is, the adjustment of the desired edge path by the operator causes the adjustment of the operating edge path such that the fabric is more deeply pinned, resulting in increased waste fabric between the position at which the fabric is pinned and the longitudinal edge.